To the Editor:—
The combined spinal–epidural (CSE) technique is performed commonly, either with use of specially designed CSE needles (needle through needle) or with use of separate needles.1When normal saline is used for the loss-of-resistance technique to locate and subsequently enlarge the epidural space in preparation for the insertion of an epidural catheter, an unusual problem may occur. We observed that the subsequent identification of subarachnoid space might be misleading because the saline used earlier for localizing the epidural space may flow into the spinal needle before it enters the subarachnoid space and be mistaken for cerebrospinal fluid (CSF). Although various methods, such as testing for glucose, protein, and pH, can be used to distinguish between CSF and saline,2,3immediate confirmation is achieved by noting the temperature of the fluid and by noting the typical “oily” appearance of the CSF as it mixes with local anesthetic (LA) solution while spinal block is being performed. However, we noticed that in a few CSE blocks, the spinal block was not effective despite this oily appearance.
To determine the reliability of the oily appearance as a means of accurately identifying CSF, we aspirated CSF and normal saline separately with 26-gauge spinal needles into glass syringes loaded with either plain or hyperbaric (containing 80 mg/ml dextrose) 0.5% bupivacaine solutions, respectively. We observed that on aspiration into hyperbaric LA solution, CSF and saline both looked oily and were grossly similar in appearance. However, this appearance was not visible with CSF or saline on aspiration into plain LA solution. The identical oily appearance with saline could be the reason for inaccurate placement of hyperbaric 0.5% bupivacaine into the epidural space, thus leading to failure of spinal anesthesia in some of the CSE blocks.
To distinguish between the two, we carefully observed oily patterns produced by CSF and saline. We noticed that CSF produced a clearly visible central and straight streak (the length of the streak was determined by the rate of aspiration, i.e. , longer on fast aspiration) that gradually moved upward (fig. 1), whereas normal saline produced a short, hazy, oily pattern that rapidly moved upward without central streak formation (fig. 2) (to highlight the oily appearance, we show CSF and saline stained with a trace of gentian violet in the figures). With the naked eye, these observations were better appreciated from the side view of the glass or transparent plastic syringe. We showed these finding to 20 anesthesiologists in our department, and all of them were able to differentiate between CSF and saline by their different oily patterns.
The typical oily appearance can be explained by the Schlieren effect, a physical phenomenon based on varying refractive indices of a transparent medium.4CSF and normal saline have refractive indices that are different from that of hyperbaric 0.5% bupivacaine solution, which contains dextrose. Hence, any change in the refractive index of hyperbaric LA solution in a glass syringe, as a result of CSF or normal saline aspiration, causes part of the light beam passing through it to be refracted, making this part of the LA solution appear brighter or darker than the rest of the background, thus causing the typical oily appearance. This oily appearance is not well appreciated by the naked eye with plain 0.5% bupivacaine solution because it does not contain dextrose and its refractive index is closer to that of CSF and normal saline.
The formation of a typical central streak with CSF and not after normal saline being aspirated into hyperbaric 0.5% bupivacaine solution can be explained by the different specific gravities of these substances. Normal saline, with a specific gravity of 1.0003, is much lighter than CSF, with specific gravity of 1.00059, which is closer to that of hyperbaric 0.5% bupivacaine (specific gravity 1.0227).5,6Hence, aspiration of saline into hyperbaric LA solution produces an oily haze that immediately floats upward, without central streak formation.
In conclusion, both CSF and normal saline produce a typical oily appearance on aspiration into hyperbaric 0.5% bupivacaine solution, and it is important to distinguish between the two by identifying the characteristic streak pattern of CSF before placing the spinal anesthetic during the CSE technique.
The authors thank Vipul Nanda, M.S., M.Ch. (Assistant Professor, Department of Plastic Surgery, Postgraduate Institute of Medical Education and Research, Chandigarh, India), for his contribution of photography.
* Postgraduate Institute of Medical Education and Research, Chandigarh, India. email@example.com